1. Field of the Invention
The present invention relates to humidification systems for gases to be supplied to a patient.
2. Description of the Related Art
For a range of applications, it is now known to be beneficial to humidify gases being supplied to a patient. These applications include where the gases are for breathing by the patient and where the gas is being supplied during surgery to the patient. In the case of breathing gases, the humidity increases patient comfort and the humidified gases are less prone to drying out the tissues of the patient airway. In the case of surgical gases, the humidified gases reduce the drying out of exposed tissue and improve post operative outcomes.
In a gases humidification system incorporating a humidification chamber for humidifying gases for supply to the patient, it is important that a certain minimum level of water is maintained in order for the humidifier to have the ability to supply water vapour to the gases flow. Accordingly, the healthcare professional administering the humidified gases to the patient, or the patient themselves, in the case of home-based administration, should occasionally check the water level and add more water when required. This task is sometimes overlooked.
U.S. Pat. No. 6,802,314 describes a respiratory humidification system which automatically determines when the water level drops to an insufficient level and raises an alarm. The system calculates, continuously, a measure of thermal conductivity as the power input divided by the difference between the heater plate temperature and the temperature of the gases exiting the humidifier chamber. The controller compares the calculated thermal conductivity with a predetermined threshold value suitable for the flow rate of gases in the system. If the thermal conductivity is less than the threshold value, the controller activates an alarm.
This respiratory humidification system is intended for an environment having fairly consistent ambient conditions. For example, hospital wards, where these devices are most frequently used, tend to be air-conditioned and maintained at a comfortable temperature and background humidity.
Another example of an alarm system for alerting a user to replenish a humidifying chamber is described in US patent application 2009/0184832. This patent application describes a humidifier for use in surgical insufflation. According to one embodiment, a controller measures the total power input to the humidifier heater over time. According to the applicant, this represents the evaporation of a proportional amount of water from the reservoir. Once the total work (accumulated power over a period of time beginning when the chamber was filled) reaches a predetermined level, the controller indicates to the user that the humidifier chamber may need refilling. In a further aspect described, the threshold level can be selected according to the temperature of the gases entering the system. According to another aspect, the system determines that hydration fluid needs to be re-charged by determining whether the instantaneous power drops below a threshold, which could be different for different flow rates or ranges of flow rates. According to another variation, the controller monitors the temperature signal of the conditioned gas approximate or inside the heater hydrator and activates an alarm when the temperature signal of the insufflation gas begins to vary significantly.
A still further system is described in U.S. application 61/289,610, filed 23 Dec. 2009 and assigned to Fisher & Paykel Healthcare Limited. This also describes a humidification system for gases used in insufflation or open-wound surgery. According to this application, a condition of low or no water in the humidifier chamber is detected by monitoring the temperature of gases exiting the chamber while monitoring the power supply to the heater base. If the temperature of the gases exiting the chamber drops at the same time as the power supplied to the heater is constant or increasing, the controller determines this as a water-out condition and alerts a user.
The surgical systems also operate within well-defined ambient conditions. For example, they are typically used in operating theatres maintained at a regulated cool temperature and in an air-conditioned environment with relatively constant ambient humidity.
The inventors consider that each of the aforementioned systems is susceptible to false alarms. They are susceptible to determining that there is an absence of water in the chamber, and alerting the healthcare professional to this condition, in situations where the chamber is not actually empty. While an accidental false alarm is not critical in a situation where the system is being used by trained healthcare professionals, they are unhelpful in situations where the humidified gases delivery systems are being used outside the controlled hospital environment. Systems used outside the controlled hospital environment may also be more susceptible to false alarms due to the wider range of ambient conditions in which they operate with ambient temperatures ranging, for example, from 12° C. to 32° C., and ambient humidity also widely varied. The varied ambient conditions are typical if the device is used in the home environment, such as devices used in CPAP therapy or oxygen therapy.